The Third Generation Partnership Project (3GPP) has adopted orthogonal frequency division multiple access (OFDMA) to achieve higher bit rates.
In cellular applications, one advantage of OFDMA is its robustness in the presence of multipath signal propagation. The immunity to multipath derives from the fact that an OFDMA system transmits information on M orthogonal frequency carriers, each operating at 1/M times the bit rate of the information signal. However, the OFDMA waveform exhibits very pronounced envelope fluctuations resulting in a high peak-to-average power ratio (PAPR). Signals with a high PAPR require highly linear power amplifiers to avoid excessive intermodulation distortion. To achieve this linearity, the amplifiers are operated with a large backoff from their peak power. The result is low power efficiency, which places a significant burden on portable wireless terminals.
Another problem with OFDMA in cellular uplink transmissions derives from the inevitable offset in frequency references among the different terminals that transmit simultaneously. Frequency offset destroys the orthogonality of the transmissions, thus introducing multiple access interference. To overcome these disadvantages, 3GPP has adopted a modified form of OFDMA for uplink transmissions in the “long-term evolution (LTE)” of cellular systems. The modified version of OFDMA, is referred to as single carrier FDMA (SC-FDMA).
As in OFDMA, the transmitters in an SC-FDMA system use different orthogonal frequencies (subcarriers) to transmit information symbols. However, they transmit the subcarriers sequentially, rather than in parallel. Relative to OFDMA, this arrangement reduces considerably the envelope fluctuations in the transmitted waveform. Accordingly, SC-FDMA signals have inherently lower PAPR than OFDMA signals. However, in cellular systems with severe multipath propagation, the SC-FDMA signals arrive at a base station with substantial intersymbol interference.
Therefore, a base station typically employs adaptive frequency domain equalization to cancel this interference when attempting to receive an SC-FDMA transmission. This arrangement makes sense in a cellular system because it reduces the burden of linear amplification in portable user equipment (UE) at the cost of complex signal processing (frequency domain equalization) at the base station.
Low PAPR is a unique property of SC-FDMA, which makes SC-FDMA very suited to use in the LTE uplink access method. Due to low PAPR, SC-FDMA is able to potentially provide larger coverage, consume less power amplifier (PA) power, and cost less, compared to, for example, OFDMA.
The current LTE standard employs SC-FDMA in the physical uplink sync channel for data transmission in the uplink. However, in order to preserve the low PAPR property, the sub-carriers assigned to a UE for SC-FDMA need to be either evenly distributed or sub-band based, i.e. sub-band of consecutive sub-carriers. The current LTE standard has adopted the sub-band based approach, so that a UE is assigned a sub-band of consecutive sub-carriers in the LTE uplink for data transmission. For the LTE UL control channel PUCCH (physical uplink control channel), the low PAPR performance is achieved by transmitting frequency domain low PAPR sequences, i.e., the sequences have low PAPR after an IFFT.
Previous research into SC-FDMA, and as a result the LTE uplink (UL), was generally focused only the use of one transmit antenna. However, it is now clear that for LTE-Advanced, which is the next step in the progression of the LTE standard, more than one antenna will be supported at UE in order to potentially improve coverage and throughput.
Although transmit diversity is well researched for OFDMA, which is utilized in the LTE downlink (DL), the technology developed for OFDMA in the DL cannot be directly transferred to SC-FDMA in the UL. This is generally due to the fact that in order to preserve the low PAPR property of SC-FDMA in the UL, the transmit diversity techniques employed in the uplink must be designed to preserve low PAPR, which is not the case for the diversity techniques developed for the OFDMA DL, as OFDMA does not have low PAPR.